The Phenom II x6 is 'its' own thing' often too has its' own set of RAM {QVL}, and I would recommend the 1600 or 1800 if OC. The resulting differences are 2-5+ FPS.
BTW - I'm working on an build now Phenom II X6 1090T and trying to find G.SKILL Flare DDR3-2000 in the US.